Fifth wheels of the type used to detachably couple the forward end of a semi-trailer to a tractor generally are attached to rail members rigidly secured to the tractor frame. This results in a rigid interconnection between the tractor frame and the forward end of the trailer. As a consequence, substantial shock is transmitted between the trailer and the tractor through the fifth wheel assembly. If a road has severe potholes or bumps in it, loads within the trailer, particularly at its forward end, often are subjected to damage as a result of the jarring shock transmitted between the wheels of the tractor and the trailer. In addition, the tractor itself is subjected to incessant pounding from the trailer through the standard rigid fifth wheel couplings. Finally, the tractor driver is subjected to the same pounding or shock throughout the time he is operating the vehicle. This creates substantial fatigue and frequently results in serious physical problems for the driver.
To alleviate the problems resulting from the incessant pounding of the tractor by the trailer and vice-versa through the fifth wheel assembly, various proposals have been made in the past. For the protection of the driver, the driver's seat in many large tractors is mounted with a resilient suspension between it and the tractor cab. Specialized suspension systems for the rear axles of the tractor also have been designed to minimize the effects of the relative pounding of the tractor and trailer through the fifth wheel assembly. Neither of these approaches, however, serve to isolate the trailer and the tractor frame from one another.
Proposals have been made for providing a resilient mounting for the fifth wheel mounting plate in the form of a support attached across the top of the tractor frame. Various types of resilient steel springs and pneumatic air bag springs have been proposed for this purpose. Most of these proposals require relatively bulky and/or complex structures which are mounted on top of the tractor frame. Examples of fifth wheel mounting assemblies of this type are shown in patents to Masser, U.S. Pat. No. 3,137,515, issued June 16, 1964; Granning U.S. Pat. No. 3,380,758, issued Apr. 30, 1968; Chalmers, U.S. Pat. No. 2,821,409, issued Jan. 28, 1958; and Granning, U.S. Pat. No. 3,253,840, issued May 31, 1966. For the most part, the relatively complex nature of the devices disclosed in these patents and the additional height added to the trailer under all conditions of use when the systems of these patents are employed, have resulted in minimal acceptance of such assemblies.
An assembly which has for its primary purpose the raising and lowering of the fifth wheel relative to the tractor frame is disclosed in the patent to Damm, U.S. Pat. No. 3,884,503, issued May 20, 1975. This patent discloses a pneumatically supported pivoted fifth wheel assembly which is mounted between the frame members of the tractor to permit a limited vertical adjustment of the fifth wheel relative to the tractor frame for the purpose of varying the forces exerted on the tractor by multiple axis semi-trailers when hilly roads are negotiated by the tractor and trailer.
A pneumatic fifth wheel suspension system which overcomes the disadvantages of those disclosed in the other patents mentioned above and which further provides the desired isolation between the tractor and trailer is disclosed in the patent to Tagg and Tourville, U.S. No. Pat. 4,279,430, issued July 11, 1981, and assigned to the same assignee as this application. The system of the Tagg/Tourville '430 patent provides a low profile assembly with a relatively simple construction which has found substantial commercial acceptance. It has been found, however, that the pneumatic air bag of this assembly (and also of the other pneumatic air bag assembly devices of the patents mentioned above) is susceptible to damage from rocks and other objects kicked up by the wheels of the tractor as it moves down the road. Such rocks and other flying objects puncture the air bag, requiring either its repair or its replacement. In addition, the fifth wheel of such assembly, and the assemblies of the other patents mentioned above, usually is mounted on rail members secured to the fifth wheel mounting plate to permit longitudinal adjustment of the location of the center of the fifth wheel for accomodating different trailers. If a particular tractor and trailer combination are to be used throughout the life of such tractor and trailer, the entire assembly may be located on the tractor frame to center the fifth wheel over the pneumatic air bag. If, however, as is frequently the case, different trailers are to be towed by the tractor, longitudinal adjustments of the fifth wheel relative to the fifth wheel mounting plate are made. Frequently, this results in the location of the fifth wheel itself at a point on the fifth wheel mounting plate either between the center of the pneumatic air bag and the pivot for the mounting plate, or off-center toward the rear of the mounting plate. In any event, uneven loading over the pneumatic air bag results, thereby impairing the optimum operating characteristics of the system.
Consequently, it is desirable to provide a fifth wheel mounting assembly which effectively cushions and isolates the tractor and trailer from communicating shock between one another during operation of the tractor-trailer, which is of simple construction, and which overcomes the disadvantages of the prior art discussed above.